Nitrous oxide, sometimes just called nitrous, is an oxidizing agent, and if delivered to an engine, can result in an increase in engine power output. It is stored in a container under pressure as a liquid in equilibrium with its vapor, thereby allowing a relatively high mass storage density. Since the vapor pressure of nitrous oxide increases with increasing temperature, the bottle pressure increases with temperature. For instance, at 0 degrees Celsius, the bottle pressure is 31E06 dynes/cm^2 (450 pounds per square inch (PSI)); when at 25 degrees Celsius, the bottle pressure is 55E06 dynes/cm^2 (815 PSI).
Nitrous oxide is commonly used as an oxidizer for engine power enhancement and as an anesthetic. When nitrous oxide is used as an anesthetic, vapor delivery systems are used wherein the nitrous oxide leaves its storage bottle as a gas or vapor. This is accomplished by placing the bottle in an upright position (assuming it does not have an internal siphon tube that connects the bottle's valve to an area near the bottle's bottom), and since the liquid is denser than the vapor and is in the lower portion of the bottle, only vapor leaves the bottle. Since the vapor is in equilibrium with its liquid, some liquid boils in the bottle to replace the lost vapor mass. A pressure regulator and orifice is commonly used to regulate the flow rate of the nitrous oxide vapor to the patient.
When nitrous oxide is used as an oxidizer for engine power enhancement, liquid delivery systems are used wherein the nitrous oxide leaves the bottle as a liquid. This is achieved by placing the bottle in an inverted position (valve down) if the bottle does not contain a siphon tube, or in a more upright position if the bottle contains a siphon tube. Liquid nitrous oxide leaves the bottle through its valve, then typically goes through a solenoid operated activation valve, through appropriate delivery lines, and finally to a nozzle which delivers the nitrous to the engine. This nozzle contains a jet or orifice which controls mass flow rate. At the entrance to the nitrous jet, the pressure is essentially the same (only slightly less) than the bottle pressure, but as it passes through the jet, its pressure decreases to typically essentially atmospheric pressure, it vaporizes, and its temperature decreases significantly. At atmospheric pressure, liquid nitrous oxide has a boiling point of −88 degrees Celsius, and this is essentially the temperature at which the nitrous oxide vapor exits the nitrous jet.
This low nozzle exit temperature of the nitrous oxide vapor is a principal reason liquid delivery systems are used for engine power enhancement (and one reason liquid systems are not used in anesthetic applications); the cold nitrous vapor is dense with a correspondingly high oxygen mass density. This can be understood by comparing the oxygen density of nitrous oxide gas at atmospheric pressure and −88 degrees Celsius (185 Kelvin degrees) to air at atmospheric pressure and a typical engine inlet temperature of 20 degrees Celsius (293 Kelvin degrees). Air is approximately 23% oxygen by weight; nitrous oxide approximately 36%. The average molecular weight of air is approximately 29; nitrous oxide is approximately 44. The oxygen density is directly related linearly to the molecular weight and the percent oxygen and inversely related linearly to the absolute temperature. The oxygen density of nitrous oxide relative to the oxygen density of air at the above conditions is therefore (0.36/0.23)*(44/29)*(293/185)=3.8. This means that if the crankcase of a two-stroke cycle engine contains a mixture of air at 20 degrees Celsius and nitrous oxide at its atmospheric pressure boiling point of −88 degrees Celsius, and if the proper fuel is supplied to both components, the nitrous oxide portion of the mix will have an energy density 3.8 times the energy density in the portion which is air.
A similar analysis can be made for nitrous oxide which is delivered as a vapor. In this case, the energy density is not amplified by the extremely cold nozzle exit temperature which exists in the liquid system. The nitrous oxide vapor cools slightly as it goes from the high pressure in the bottle to atmospheric pressure, but this temperature difference is relatively small and will be ignored for this discussion. Assuming the air and nitrous oxide are at the same temperature, the relative oxygen density of nitrous oxide to that of air is represented by (0.36/0.23)*(44/29)=2.4.
Therefore, this analysis shows that a liquid nitrous oxide system used for engine power enhancement will have an inherent advantage over a vapor system due to the potentially higher oxygen and energy densities attainable with the liquid system. Therefore, nitrous oxide delivery systems used as engine oxidizers have been liquid systems.
The above analysis assumes an ideal environment, but the real world is different. Things can occur which actually lessen the seemingly insurmountable advantage of a liquid delivery system over a vapor system in most applications. First, although the nitrous oxide vapor delivered by a liquid system initially has a temperature of −88 degrees Celsius as it leaves the nitrous nozzle, this extremely cold temperature is not maintained if the nitrous oxide vapor must travel through the engine's induction tract. Its temperature will increase due to the extremely large temperature difference between the cold vapor and warmer engine components. Another problem with liquid nitrous oxide systems is the fact that the nitrous oxide actually leaves the nozzle as a mixture of vapor and ice crystals. These ice crystals are abrasive and can cause engine damage. Also, if the vaporizing nitrous oxide is sprayed where it relatively quickly contacts a solid surface, such as the interior of an engine or its induction tract, there is a deposit of nitrous oxide ice. This ice deposit prevents the total immediate use of the nitrous oxide as an oxidizer, therefore lessening its effect on engine power, and the ice can break off and go through the engine and cause damage. If sprayed on or near reeds commonly used as induction valves for two-stroke-cycle engines, reed breakage is common due to the extremely cold temperature of the spray. Another problem with liquid nitrous oxide systems is the fact that the relatively high pressure existing in the bottle is essentially maintained until the nitrous oxide exits the nozzle, requiring high pressure lines to be used for the entire delivery system. Also, if a liquid nitrous oxide system is used for small horsepower engines which require a relatively small mass delivery rate of nitrous, the jet which controls the nitrous oxide flow becomes very small. For liquid systems required to deliver a low flow rate of nitrous, Inventor has had to use nitrous oxide jets as small as 0.15 mm (0.006 inches) in diameter, these extremely small orifice sizes being expensive and difficult to manufacture.
Also, in liquid delivery systems, the components of the system downstream of the bottle, such as the activation valve and delivery lines, are commonly warmer than the liquid in the bottle due to their location relatively close to the engine and exhaust. Boiling of liquid nitrous will occur in these components to cool them, resulting in a mixture of liquid and vapor. This causes less than rated nitrous delivery through the nitrous jet due to the lower density of the mixture relative to a “pure” liquid, and contributes to an undesirable effect called nitrous delay. Purge valves, valves which vent nitrous to the atmosphere, are used to remove this nitrous vapor to insure relatively “pure” nitrous liquid exists in the system at the start of actual system use.